Fiber Bragg gratings for reflecting (or transmitting) wavelengths of light within optical waveguides are known to have reflectance functions that suffer from undesirable side lobes when the grating has a constant coupling strength along its length. Such side lobes are especially undesirable for narrow band and dense wavelength division multiplexing. Problems with such side lobes and attempts at eliminating such side lobes have been discussed in U.S. Pat. Nos. 5,309,260 and 5,388,173, for example.
For many purposes, the optimum reflectance function for a fiber Bragg grating is a three-sided rectangle, with a flat top and nearly vertical sides in a "tophat" configuration. Prior art suggestions for different ways of making fiber Bragg gratings have so far failed to achieve this.